It was a mistaken assumption from the Homebrew developers who didn't realize AVX would cause problems with Rosetta2.

Within a few days problems became evident and Homebrew turned the compiler flag back off. Now it's just a question of the best way of remediating the already-compiled packages in the simplest way possible.

Really the fact that this is now a front-page-of-HN story is going to cause more confusion to people. It was just a short-lived bug.